Enhancement of magnetic field effect in Ru(bpy)3 2+ / MV 2+ system by Ru(bpy)3 2+ -Ag+ exciplex formation

Ž . Chemical Physics Letters 316 2000 411–418

www.elsevier.nlrlocatercplett

ž /

Enhancement of magnetic field effect in Ru bpy

r MV

ž /

system by Ru bpy

-Ag exciplex formation

Lajos Fodor

, Attila Horvath ´

, Karsten A. Hotzer ¨

, Stefan Walbert

, Ulrich E. Steiner

aDepartment of General and Inorganic Chemistry, UniÕersity of Veszprem, Veszprem, H-8201, P.O.B. 158, Hungary´ ´

bFakultat f ur Chemie der UniÕersitat Konstanz, D-78457 Konstanz, Germany¨ ¨ ¨ Received 12 July 1999; in final form 7 October 1999

Abstract

The influence of ionic strength variation and of exciplex formation between silver ions and triplet excited RuL²₃^q ŽLsbipyridine, phenanthroline on the photo electron transfer kinetics of the complex with methylviologen and the. magnetic field dependence of free radical formation efficiency has been studied by laser flash spectroscopy. The magnetic

Ž .^{2q 2q}

field effect consisting in a decrease of the efficiency of cage escapeh_ce in the Ru bpy ₃ rMV system is enhanced by 40% when exciplex formation between Ag^qions and triplet excited RuL²₃^q complexes takes place. The relevant kinetic parameters have been determined by magnetokinetic model calculations.q2000 Elsevier Science B.V. All rights reserved.

1. Introduction

Light induced electron transfer reactions from tris-diimineruthenium II complexes to various elec-Ž .

Ž ^2q. tron acceptors such as methylviologen MV have been extensively studied because of their potential applicability in the design of artificial systems for

w x conversion and storage of solar energy 1–7 . Partic- ular attention has been paid to the kinetics of gemi-

Ž .

nate backward electron transfer GBET representing an energy loss channel that competes with the forma- tion of energy-rich free radicals. The discovery of a responsiveness of the yield of free redox products in

) Corresponding author. Fax: q49-7531-88-3014; e-mail:

steiner@chclu.chemie.uni.konstanz.de

1Also corresponding author. E-mail: attila@vegic.sol.vein.hu.

w x such reactions to high magnetic fields 8,9 has provided a useful technique in elucidating the mech-

w x

anism of the GBET process 10–15 .

Ž .^{2q 2q}

In the Ru bpy ₃ rMV system the magnetic

Ž .

field effect MFE becomes measurable in fields of some 0.1 T and reaches a saturation limit in fields of about 10 T. It consists in a decrease of the efficiency of free redox product formation by up to several

³w x^{2q 2q}4 10%. MFE studies of RuL₃ . . . MV reac- tions have revealed a high sensitivity of the MFE to

w x

ligand variation 10,15 and to solvent viscosity vari- w x

ation 16 . This type of MFE has been accounted for

w x

by the spin chemical radical pair mechanism 17,18 which is based on the rule of spin conservation in fast chemical reaction steps, demanding that a spin change has to occur before recombination can take place if the multiplicity of the spin state in which the

0009-2614r00r$ - see front matterq2000 Elsevier Science B.V. All rights reserved.

Ž .

PII: S 0 0 0 9 - 2 6 1 4 9 9 0 1 3 0 7 - X

Konstanzer Online-Publikations-System (KOPS) URL: http://www.ub.uni-konstanz.de/kops/volltexte/2008/4738/

URN: http://nbn-resolving.de/urn:nbn:de:bsz:352-opus-47387

3RuL2₃ <Ag . and ternary as RuLŽ ^{3 2}₃ <Ag²₂ . exci- plexes in the presence of inorganic ground state

q w x w x

species such as Ag 20–23 and HgCl₂ 24 , re- spectively. In aqueous solution, addition of silver

Ž .

ions up to 1.5 M at constant ionic strength 3 M results in the decrease of the lifetime and the lumi-

3 2q

nescence quantum yield of RuL₃ ŽLsbipyridine or phenanthroline and a shift of the emission band.

w x ^2q

to longer wavelengths 25 . With MV these exci- plexes undergo an electron transfer reaction similar to the parent Ru II complex. In case of the binaryŽ . and ternary exciplexes mentioned above both the rate constant k_q of quenching by MV^2q and the effi- ciencyh_ce of free radical formation in the quenching process decrease. Whereas the decrease of the quenching rate constant may be assigned to the higher positive charge of the exciplex, since it re- tards the approach of the MV^2q quencher dication, the origin of the decrease of the cage escape effi- ciency might be sought in a faster GBET andror a slower cage escape process of the RP. In order to get more insight into this question the present MFE study was undertaken.

2. Experimental

2.1. Materials and solutions w Ž . x

Analytical grade Ru bpy ₃ Cl₂P6H O₂ and wRu phenŽ .₃xCl₂Px H O Aldrich , AgNO₂ Ž . ₃ ŽReanal ,.

Ž .

NaNO₃ Reanal were used without further purifica-

2.2. Equipment and methods

Transient emission and absorption signals in the nanosecond to microsecond range were measured in different external magnetic fields up to 3.3 T mag- netic flux density using the laser flash photolysis

w x

equipment described previously 27 . Coumarin 47 was used as laser dye to obtain a wavelength of 470 nm for exciting the ruthenium complexes. The dye was pumped by the third harmonic of a Nd:YAG

Ž .

laser Spectra Physics, Quanta Ray GCR 150 . Lu- minescence lifetimes were obtained by a single-ex- ponential fitting to the emission decay Ž´_ems600 nm . The efficiency of net electron transfer,. h_ce was determined by the saturation method, detecting the MV^qØ absorption at 395 nm as a function of the

w x laser energy and extrapolating to saturation 28 . The efficiency of net electron transfer was obtained using the following equation.

hcesDOD395 ,satr

Ž

.

where DOD_{395, sat} is the saturation value of absorp- w x Ž w x. tion at 395 nm, h_qsk Q_q r1rt_oqk Q_q is the

w Ž .x

quenching efficiency, Ru II _o is the total concentra- tion of ruthenium complexes, D´ is the change of

Ž .

molar absorbance D´s´_MVqq´RuŽIII.-´RuŽII. . The quenching constant k at various ionic strengths and_q Ag^q concentrations was measured by the lumines- cence quenching method using the Stern–Volmer

w x

equation: 1rts1rt_oqk Q , where_q t_o is the life- w x

time of the luminescent species at Q s0, k is the_q

Scheme 1.

w x rate coefficient of the quenching reaction and Q is the concentration of the quencher.

3. Results

The reaction scheme describing the system inves- tigated is presented in Scheme 1. Association be-

q Ž .

tween Ag ions and the Ru II complex occurs only when the complex is in the excited state, whereupon bimolecular and termolecular exciplexes are formed.

In order to achieve high efficiencies of exciplex formation 1.5 M solutions of Ag^q were employed, yielding equilibrium concentrations of 7, 35, and 58% of free ruthenium complex, bimolecular and termolecular exciplex, respectively. It is important to note that due to the high silver ion concentration the time to establish the exciplex equilibrium is much shorter than the quenching reaction with MV^2q at

w x any quencher concentration employed 25 .

The results of our measurements of the parame- ters of the electron transfer reaction with MV^2q, viz.

Table 1

3 Ž .^2q

Characteristic reaction parameters measured and evaluated for the primary radical pair formed in the electron transfer reaction Ru bpy3

2q w ^qx w ^qx

qMV in aqueous solution at different ionic strength for Ag s0 and Ag s1.5 M

2q 2q 2q 2q 2q q 2qa

Ž . Ž . Ž .

System parameter Ru bpy3 yMV Ru bpy3 yMV Ru bpy3 yAg yMV

w x

ISs0.1 M ISs3.0 M ISs3.0 M, Ags1.5 M

measured quantities

y9 y1y1 b

Ž .

kq=10 , M s 0.85 0.48 2.7 1.5

Ž .b

h_ce, % 15.5 14.7 6.4 2.9

Ž .b

R_hce, % measured at Bs3.3 T 19 23 17.5 25

evaluated quantities

Ž .b

t_s, ps 21 24.4 23 27

y1 b

Ž .

k , ns_ce 2.1 1.7 0.45 0.29

y1 Ž .b

k_bet, ns 83 65 42 87

a 3

Ž .^2q<Ž ^q.

The values given are average values refering to a mixture of species Ru bpy3 Ag nwith 7, 35, and 58% for ns0, 1, 2.

bData from Ref. 27 .w x

low fields and a slight decrease at high fields. Upon exciplex formation with silver ions k_q slightly de- creases whereash_ce is reduced by a factor of about 2. The MFE on h_ce is considerably enhanced at all fields investigated, its increase amounting to about 40% at 3.3 T.

Ž .

Fig. 1. Relative magnetic field effect R B0 on the quantum yield of MV^qØ radical formation upon quenching of photoexcited

2q Ž . w ^qx

RuL3 complexes: ` Lsbpy, ms0.1 M, Ag s0 M;

ŽØ.xLsbpy,ms3.0 M, Agw ^qxs0 M;ŽB.Lsbpy,ms3.0 M, wAg^qxs1.5 M. Inset represents R BŽ 0. data obtained for the

Ž . w ^qx Ž .

Lsphen complex: I ms3.0 M, Ag s0 M, Bms3.0 w ^qx

M, Ag s1.5 M.

ternary exciplex in similar amounts the results ob-

Ž ² :.

tained represent averages below denoted by of the behavior of binary and ternary exciplex. At the present stage of investigation, no attempt has been made yet at differentiating the two exciplex species, although, in principle, this is possible by investigat- ing in detail the Ag^q concentration dependence.

As shown in Scheme 1 electron transfer quench-

3 Ž .

ing of the Ru II species generates pairs of redox products each with an overall triplet alignment of the unpaired spins. Before the redox pairs can undergo geminate recombination occuring with rate constant

k_{b e t} they have to undergo spin transitions generating

singlet spin alignment. In zero magnetic field such spin transitions occur only incoherently through spin

Ž . Ž

relaxation at the Ru III centers. The primes used with the multiplicity symbols 3^X and 1^X denote some spin-orbit-coupling induced contamination with com- ponents of the other multiplicity . Spin equilibrium is. established with a rate constant of t^y_s¹, t_s denoting

Ž .

the transversalslongitudinal and field independent Ž .

relaxation time of the Ru III complex. In an external

Ž .

magnetic field an additional coherent mechanism of T–S transitions based on the different g tensors of the two types of redox species comes into effect.

In competition with spin-dependent backward elec- tron transfer the geminate pairs of redox products

Ž .

may dissociate k_ce into the free redox products.

The efficiency h_ce of this ‘cage escape’ process is measured in the experiment. It should be noted that in Scheme 1 the energetic separation of the different spin multiplicities of the redox-pairs and the free redox products is not drawn to scale. It is important

to note that these states have practically the same energies.

First we discuss the observed effects on k . As_q

w x

has been shown by Clark and Hoffman 29–31 Ž .^2q electron transfer rate constants in the Ru bpy r

3

MV^2q system are enhanced by ion-pairing of these cations with anions of the added salts. This enhance- ment was reported to correlate with the free enthalpy DG^o_hydr of hydration of the anion: the less negative DG^o_hydr the faster the electron transfer with ruthe- nium complexes paired with these ions. This correla- tion was suggested to be due to a smaller reorganiza- tion energy l_solv associated with anions of higher

Ž ^o . ^o

hydrophobicity less negative DG_hydr . The DG_hydr

y Ž

values of NO₃ not investigated by Clark and Hoff- man of. y1362 kJrmol is very close to the value of

yŽ .w x

Br y1377 kJrmol 29,32 . In agreement with the correlation noted by Clark and Hoffman the k_q val- ues forms0.1 M and 3.0 are compatible in order of magnitude and trend of concentration dependence

Ž ⁹

with the values reported for NaBr 1.88=10 M^y1s^y1 for ms0.1 and 3.96=10⁹ M^y1 s^y1 for ms1.0, the highest concentration investigated by these authors. But here the salt concentration depen-

w x. dence of k seems close to saturation 29 ._q

While k_q the rate constant of the bimolecular forward electron transfer is obtained as a direct result of the kinetic experiment, the rate constants k_bet and k_ce are not directly observable but can only be recovered from the actual observable h_ce and its

Ž .

magnetic field dependence R B . The procedure for_o this evaluation is based on kinetic simulations em- ploying the time-integrated solution of the Stochastic Liouville equation for the spin density matrix r of the geminate radical pair as described in detail in

w x

Refs. 11,13,19 . Apart from the g-tensor values of the radical, known from ESR data Žfor the

Ž . w x

Ru III complex g5s1.18, gHs2.60 15 , for

qØ .

MV a value of gs2.00 can be used the spin chemical model involves three kinetic parameters, k , k_{ce bet}andt_s corresponding to the rate constants of cage escape, fully spin-allowed backward electron

Ž w x.

transfer and transversal equal to longitudinal 13 Ž .

spin relaxation time of the Ru III complex, respec- tively.

For the details of the spin chemical theory and the numerical calculations we are referring to previous

w x

work 11–13,19 . Here we will only briefly describe

and rationalize the influence the three kinetic param- eters have on the observable quantities h_ce and

Ž .

R B . For situations where₀ h_ce<1, the case pre- vailing here, the geminate radical pairs mainly decay through backward electron transfer. Therefore it is clear that in this case k_ce<k_bet, 1rt_s. In such a situation any change of k_ce will be strongly reflected in the relative change of h_ce, but have only little

Ž . effect on the magnetic field dependence R B . The₀ latter conclusion follows from the fact that a mag- netic field can only exhibit its effect during the lifetime of the geminate pair on which, however, k_ce has little influence if k_ce<k_bet, 1rt_s. On the other hand, for this kinetic situation k_bet andt_s do have a strong effect on the lifetime of the geminate radical pair and hence on the magnetic field effect. From these general dependences the following strategy can be derived for obtaining a unique set of parameters

Ž . k , k_{ce bet} andt_s to fit the observedh_ce and R B .₀

First k_ce is set to a limiting small value, such that the MFE is essentially independent of any further reduction of k . Then k_{ce bet} andt_s are varied subject to the restraint that the observed R value for the

Ž .

highest field is correctly reproduced cf. Fig. 2 . Thereby a family of curves is obtained differing in shape but all crossing in a common point. The suitability criterion of the model requires that one of

4

Fig. 2. Theoretical curves representing the influence of k_{b e t},t_s

Ž . ^y1

on R B . Parameter k_{0 c e} was kept constant at 0.45 ns . The sets

4

of kb e t,ts values were selected with the restraint to reproduce

Ž .

the experimental value of R 3.3 T . The pertinent values for

Ž . 4 4

curves 1– 6 numbering from above are: 1: 153, 23 , 2: 111, 25 ,

4 4 4 4

3: 90, 27 , 4: 76, 29 , 5: 67, 31 , 6: 60, 33 . The dashed curve

Ž .

represents the negligible change of curve 3 after setting kc e to

y1 Ž .

0.29 ns , yielding the experimentalhc e value of 2.9%; Ø are w ^qx

the measured data for Lsbpy,ms3.0 M, Ag s1.5 M.

w x for NaCl solution in one of our laboratories 27 . The magnetic field effect curve R B in NaNO solutionŽ . ₃ is also similar to what has been found earlier in NaCl

Ž .

solution cf. Table 1 . Accordingly, the resulting values for t_s, k_ce and k_bet are similar to those in previous work with NaCl solutions. Comparing our k_ce and k_bet values with those published by Clark and Hoffman it is important to note how their values were assessed. Clark and Hoffman estimated k_ce by

Ž .

employing the Eigen–Debye ED equation which

Ž . ^{9 y1}

yielded a value of k_ce ED s5.8=10 s . Albeit on the same order of magnitude as our value it still differs from it by a factor of almost 3. For the understanding of this discrepancy it is interesting to note that there is some principal ambiguity in relat- ing results from continuous diffusion theory to the parameters of a kinetic model comprising only first

w x

order rate processes 33 . For example, in theoretical simulations comparing the two types of models dif-Ž fusional and ‘exponential’ for redox pair recombina-. tion processes with the same spin characteristics as

w x

in the present systems 27 it was found that the equivalent first order rate constant of cage escape was typically 2 times smaller than the ED value.

Ž .

Using the k_ceED value given above Clark and Hoffman evaluated k_bet from h_ce by employing the relation:

h_cesk_cerŽk_ceqk_bet. . Ž .2

Ž .

In doing so the role of spin processes cf. Scheme 1 is completely neglected. Therefore the resulting k_bet value should be considered as an ‘apparent’ value k_bet,app. Their result of 3.2=10¹⁰ s^y1 is 2.5 times

it is close to the k_bet,app value estimated from Eq. 2Ž . when our spin-chemically consistent k_ce value is used. This agreement demonstrates that the BET process is actually spin-conversion controlled. The true k_bet value is about 8 times larger and can only be assessed with the help of the MFE.

When increasing the ionic strength from ms0.1 to 3.0 M a significant decrease of h_ce is observed Žcf. Table 1 . On the other hand, the MFE curve. changes only little with a minor enhancement of the MFE at lower fields and a minor attenuation of the MFE at higher fields. From our general reasoning given above on the dependence of the MFE curves on the various kinetic parameters it should be quali- tatively clear that k_ce must undergo a major change.

It decreases by a factor of 4.5 while k_bet is halved and t_s is nearly constant. In solutions with salt concentrations as high as 3.0 M, which are clearly out of the validity range of Debye–Hueckel theory, it is questionable to apply the ED equation for the interpretation of k . Employing this equation never-_ce theless and accounting for the 23% higher viscosity w34 leads us to expect that kx _ce should drop by no more than a factor of 2. On the other hand, in highly concentrated salt solutions the Coulombic shielding effect on the ionic cloud which leads to a decrease of k_ce for reactants of like charge, as in the case of Ru^3qrMV^qØ, is counteracted by short distance forces between ions in contact. Furthermore the dielectric

w x constant is somewhat depressed by the salt 35 . Both these effects should lessen the decrease of k ._ce Therefore we do not favor the possibility to explain the assessed k_ce effect by the bulk properties of the

solution. A specific binding of Ru^3q and MV^qØ moiety by microscopic structural changes of their solvent and ionic spheres seems more reasonable.

These structural changes must be adverse to electron transfer, too, as to be concluded from the evaluated decrease of k_bet. To which of the three basic parame- ters of electron transfer theory thermodynamic driv-Ž ing force, reorganization energy, and electronic cou- pling matrix element this change of k. _bet should be assigned is difficult to assess, however, with the information at hand.

The fair constancy of t_s is in line with the w x

previous notion 13 that the spin relaxation of the Ru^III complex is rather due to an intramolecular perturbation vibrational fluctuations of the ligandŽ field, indirectly modifying the spin-orbit coupling. involving primarily the inner ligand sphere of Ru^III ion.

Finally we have to discuss the effects introduced

3 Ž .^{2q q}

by exciplex formation of Ru bpy ₃ with Ag ions. Replacing 1.5 M Na^q ions in the solution by

q ² :

Ag ions leads to a further decrease in h_ce by a factor of 2 and a clear enhancement of the MFE.

According to the spin chemical analysis these changes

² :

are related to a further decrease of k_ce , a doubling

² : ² :

of k_bet and a slight increase of t_s . Since there is unequivocal indication of exciplex formation, Ag^q must be in specific association with the MLCT ex-

Ž .^2q w x

cited Ru bpy₃ species. It is likely 25 that the electron promoted to a bpy ligand in the MLCT transition is delocalized to some degree over the Ag^q ion in the exciplex. This will certainly lower

Ž ⁾.

the LUMO bpy p energy thereby reducing the thermodynamic driving force of the forward electron transfer which qualitatively explains the observed

² :

decrease of k_q . While it will not directly affect the driving force of the backward electron process, the lowering of the LUMO might be still favorable for a faster backward electron transfer by enhancing the backward electron transfer related electronic cou-

qŽ . pling via an enhancement of the MV SOMO ™

Ž ⁾. Ž .

bpyp ™Ru d superexchange mechanism.

5. Conclusion

We have shown that magnetic field effects are indispensable for unravelling the details of backward electron transfer and cage escape kinetics in

Ž .^{3q qØ}

Ru L ₃ rMV type redox pairs characterized by very fast spin relaxation. The spin chemical analysis of the cage escape efficiency and its magnetic field dependence has revealed that exciplex formation be-

q 3

Ž .^2q Ž

tween Ag and Ru L ₃ Lsbipyridine, phenan- throline significantly reduces the rate constant of. cage escape and increases the rate constant of back- ward electron transfer in the photo-induced reaction of the Ru-complex with methylviologen.

Acknowledgements

Financial support by the Hungarian National Sci-

Ž .

ence Foundation OTKA No. T23760 and by the priority program Inter- and Intramolecular Electron Transfer of the Volkswagenstiftung is gratefully ac- knowledged.

References

w x1 N. Sutin, C. Creutz, Pure Appl. Chem. 52 1980 2717.Ž . w x2 K. Kalyanasundaram, Coord. Chem. Rev. 46 1982 159.Ž . w x3 K. Kalyanasundaram, in: M. Gratzel Ed. , Energy Resources¨ Ž .

through Photochemistry and Catalysis, Academic Press, New York, 1983.

w x4 V. Balzani, F. Scandola, in: M. Gratzel Ed. , Energy Re-¨ Ž . sources through Photochemistry and Catalysis, Academic Press, New York, 1983.

w x5 M. Venturi, Q.G. Mulazzani, M.Z. Hoffman, J. Phys. Chem.

Ž .

88 1984 912.

w x6 C. Chiorboli, M.T. Indelli, M.A. Rampi, F. Scandola, J.

Ž .

Phys. Chem. 92 1988 156.

w x7 M.Z. Hoffman, J. Phys. Chem. 92 1988 3464.Ž .

w x8 G. Ferraudi, G.A. Arguello, J. Phys. Chem. 92 1987 1846.¨ Ž . w x9 U.E. Steiner, H.-J. Wolff, T. Ulrich, T. Ohno, J. Phys. Chem.

Ž .

93 1989 5147.

w10 H.-J. Wolff, U.E. Steiner, Z. Phys. Chem. N.F. 169 1990x Ž . 147.

w11 U.E. Steiner, D. Burßner, Z. Phys. Chem. N.F. 169 1990x ¨ Ž . 159.

w12 U.E. Steiner, W. Haas, J. Phys. Chem. 95 1991 1880.x Ž . w13 D. Burßner, H.-J. Wolff, U.E. Steiner, Z. Phys. Chem. N.F.x ¨

Ž .

182 1993 297.

w14 M. Mukai, H. Tanaka, Y. Fujiwara, Y. Tanimoto, Bull.x

Ž .

Chem. Soc. Jpn 67 1994 3112.

w15 D. Burßner, H.-J. Wolff, U.E. Steiner, Angew. Chem. Ed.x ¨

Ž . Ž .

Engl. 33 17 1994 1772.

w16 H.-J. Wolff, D. Burßner, U.E. Steiner, Pure Appl. Chem. 67x ¨ Ž1995 167..

w17 K.M.x Salikhov, Y.N. Molin, R.Z. Sagdeev, A.L.

Buchachenko, Spin Polarization and Magnetic Effects in Radical Reactions, Elsevier, Amsterdam, The Netherlands, 1984.